On April 7, two months after she was sworn in as assistant to President Carter for public liaison. Margaret (Midge) Costanza and some friends threw a party.

It was a $500-a-ticket fund-raiser with which she hoped to wipe out a $17.615 deficit lingering from her unsuccessful 1974 campaign for Congress.

All but $570 of the debt was owed to Costanza herself. The proceeds, she explained were "to repay me" for money she put into her 1974 cmapaign.

Vice President Mondale was the featured attraction and, as is the case in such affairs, tickets were purchased by Costanza's old friends and groups and individuals who had an interest in helping a new senior White House aide with a financial problem.

"I needed my money," Costanza said yesterday in discussing the affair. "I had relocated to Washington and didn't have any money and I was very eager to pay me back."

By her own acount, Costanza raised about $21.025. When the money came in, she said."I paid me."

She said she is now trying to determine what she can legally do with the $3,400 surplus.

As of yesterday, neither Costanza nor her campaign committee had filed reports with the Federal Election Commission on the April party or any other funds raised or spent between April 1 and June 30.

Under the law, those reports should have been delivered to the commission by July 10.

"I'm not aware of anything impoper," Costanza said yesterday, "other than [I have] not filed . . . but I have a good reason for that," she added, citing the fact that she is trying to find out what can be done with the surplus.

She said a month or more ago she called the FEC to find out if she can use the surplus to pay Sandra M. Ajams, her White House administrative assistant and her campaign trea- surer for her work in making out the campaign reports.

"I wanted to get a bill from her for her services and pay her," Costanza said. She also wants to find out if the surplus funds can be used to pay for Adams' trip to Rochester N.Y., to help solicit donations for the Costanza party.

Finally, Costanza said she wrote a $150 check to the campaign of Carol Bellamy for New York City Council President and wanted to reimburse herself from the fund for that donation. It was something I could not afford personally," she said.

Although her report is not filed Costanza yesterday read off the list of contributors in a telephone interview. Among them were six labor unions including the United Auto Workers ($1,000): the communications workers ($1,300) and the Brooklyn longshoremen ($1,000).

Costanza bristled when questioned about the lack of FEC reporting and the propriety of a White House aide holding a fund-raiser to pay off past campaign debts to herself.

"I'm not a wealthy woman," she said yesterday. "The kind of money I put in [the campaign] was more than I could afford to lose. I always had the idea I would raise it. It wouldn't make any difference whether I was in the White House or not."

As for the labor contributions. Costanza said, "I don't deal with labor issues in this office." They were solicited because the unions "supported me in the campaign" and in 1974 encouraged her, she said, "to make the special investment" of her own money, "from which the debt arose."

Costanza grew particularly irritated when asked about the failure to file FEC reports on her fund-raising.

"I called them," she said, "and told them I'd be a little late . . . They didn't think they should give [an extension] to me because of who I am. Can you believe that?"

An FEC spokesman confirmed that Costanza had called the commission several times to say her initial 1977 filing, due April 10, would be late. The commission, according to the spokesman, waited until May 20 for the report and then sent Costanza a notice requesting it.

On May 31, Adams, Costanza's White House assistant and campaign treasurer, wrote the commission enclosing the April 10 report, which indicated that no funds had been raised.

In that letter, Adams also wrote: "For the record, this committee held a fund-raiser on April 7, 1977, and raised approximately $20,000."

She went on to say all checks from the event had not come in and added.

"I do anticipate receiving it by the next filing quarter."

The July 10 filing date passed and no report was sent. The FEC, however, never sent Costanza another notice. The commission spokesman said yesterday that lack of manpower prohibited the FEC staff from being able to keep track of non-filers.

Costanza said yesterday she was pulling together "everything the FEC needs or wants" to make her final filing. "I called them just two weeks ago," she said, to say her report terminating the committee was in the works.

Costanza said yesterday White House officials were "aware I had to pay off a debt" when she joined the staff. She said she asked White House Coursel Robert Lipshutz "to relax the rule" that prohibited presidential aides from fund-raising for themselves.

Lipshutz said yesterday there was no such rule but that he had turned Costanza's situation over to Mondale's counsel. Michael Berman, since the Vice President was to be the guest of honor at the fund-raiser.

Berman said he "established the debts existed" and were legitimate. He also determined "now law precluded" holding such an event. He said told Costanza and Adams "they couldn't solicit federal employees" and to avoid donors "that would cast doubt on the integrity of the White House."

Berman said yesterday that was all he did - that he had never looked into who donated or whether Costanza had filed her reports with the FEC.

Lipshutz said the White House had discussions on two other campaign-fund questions since the Carter administration took office.

One was over possible fund-raising to wipe out 1974 gubernatorial campaigning debts of former Office of Management and Budget Director Bert Lance. There, Lipshutz said, they could not determine a way a fund-raiser could be held, so the idea was suspended.

In the case of a nominee to the Interior Department who has a $150,000 city council campaign debt, the administration worked out a procedure whereby the chief lawyer from that department reviewed all the contributions for possible conflict of interest and then published each one accepted.

Asked why such a procedure was not followed in Costanza's case, Lipshutz said yesterday that he thought the matter was being handled by Berman.